Determining the soil characteristics of the seabed is an essential part of the geotechnical design of offshore installations, which can include structures for oil and gas developments, seabed anchorages, submarine pipelines and cables, and wind energy developments. The in situ properties of seabed soils profoundly influence the design and performance of structures that interact with the seabed. The trend in the offshore energy industry towards working in deep and ultra-deep water, where soft sediments predominate, places increasing importance on the ability to accurately measure soft soil strength and deformation parameters. Examples of prior art documents describing known techniques for seabed analysis include U.S. Pat. Nos. 5,127,261; 6,463,801 and 6,526,818.
Sediment strength profiles are commonly measured in situ using a variety of tools, deployed by various means including ‘wireline’ drillstring, coiled ‘wheel-drive’ tubes, seabed push frames and more recently by remotely operated seabed platforms. Tools commonly used to profile the shear strength of soft cohesive sediments include the cone penetrometer test (CPT) and vane shear test (VST). A relatively recent penetrometer device, known as the ‘T-Bar’, has also been deployed in the field. The T-Bar device has advantages over the conventional CPT and VST tools, in particular a closed form solution exists for undrained shear strength as a function of bar penetration resistance, and the geometry of the device also negates the need to correct for water pressure effects. In soft soils, these factors combine to reduce uncertainty in design and significantly enhance the resolution of the measured soil strength profile. However, all of the above tools have limitations.
Key disadvantages with the use of CPT devices in soft soils include:                a) cone correction factors, which are used to convert CPT tip resistance to soil shear strength, are empirically based and vary widely, depending on the type and state of the soil undergoing test. Uncertainties associated with the application of cone factors typically leads to a high degree of conservatism in design;        b) CPT devices (particularly when subjected to high ambient water pressures), require a correction for the effects of water pressure acting on the unequal areas of the cone tip. This correction is typically very significant in soft sediments, which mobilise relatively low cone tip pressures and high excess pore water pressures;        c) cone penetrometers have a relatively poor capacity to accurately profile soft sediments. They have relatively small tip areas (reducing the load mobilised on the cone tip load cell to typically less than a few MPa), and relatively high capacity load cells (sediments that mobilise tip pressures of 50 MPa or greater can usually be probed). These factors combine to reduce the available resolution of the device.        
Disadvantages of the VST include:                a) the VST cannot provide a continuous measurement of the shear strength profile with depth;        b) the VST is not suitable for measuring the shear strength of dilatant, coarse grained soils;        c) knowledge of the soil type is required for correct interpretation of VST results.        
A key disadvantage of the T-Bar penetrometer for deployment via a drillstring of relatively small diameter, lies with the required geometry of the horizontal bar. Research to date indicates that an aspect ratio (length/diameter) of typically 4 to 8 is required for accurate in situ measurement of soil strength. To provide sufficient bearing area of the bar, this means that bar lengths of more than 150 mm are usually required. This presents problems for many offshore drilling units, particularly remotely operated seabed drilling units, which currently are incapable of handling these large dimension tools.
The spherical ball penetrometer (SBP) is a known alternative device that offers the same fundamental benefits of the T-Bar, but without the associated problems of geometry as described above. The SBP offers a number of distinct advantages over conventional in situ test methods, which include the following:                a) there exists an exact solution for shear strength as a function of ball bearing resistance;        b) the ball bearing area is significantly greater than for a standard CPT tool—this means that much higher bearing forces are mobilised in soft sediment profiles, greatly enhancing the resolution of the measured shear strength profile;        c) there are no significant water pressure corrections required, simplifying data processing and reducing uncertainty of the measurements;        d) a measure of remoulded and cyclic shear strength degradation is also possible via static withdrawal and or cyclic loading of the tool following penetration into the soil.        
Despite these advantages the SBP test has not been adopted for general field use and the present applicant is not aware of any published reports of its deployment in an offshore environment. SBP probes disclosed in the literature are individual dedicated instruments with simple built-in load cells close to the ball, with no capability for optional measurement of pore water pressure. Furthermore, presently known SBPs rely on wired electrical connections between the downhole probe and the surface equipment for power supply and data telemetry.
It would be advantageous to provide an improved ball penetrometer probe that is interchangeable with conventional CPT probes or the like, and has a reasonable cross sectional geometry suitable for both wireline deployment and for deployment using remotely operated seabed systems. In the latter case, such a tool can exploit the use of wireless data transmission, such as known acoustic methods, to transmit measurement data from the downhole probe to operators for real-time analysis and display. It is preferable to provide wireless data transmission so that the probe can be usefully employed on a seabed system that relies on remotely joining discrete lengths of drill pipe to advance the probe into the seabed soil formation.
As used herein, the phrase “remote operating station” generally refers to a surface vessel or platform, where the downhole data is ultimately received by a computer interface and human operator. In the case of a wireline system the remote operating station is connected directly to the downhole probe by wire(s) and/or cable(s) through the water column and the borehole. Another alternative for so-called “measurement while drilling” uses a “mud pulse” system that transmits data via pressure pulses in the drilling fluid up the drill string, however this is impractical for small diameter tools.
Also as used herein, the phrase “remotely operated seabed system” generally refers to the situation where the probe is deployed robotically or otherwise down the borehole from a seabed platform or other type of vehicle rather than manually from a surface platform. Communication from the probe to the seabed platform/system may be by wire(s), cable(s) and/or by wireless means. Communication between the seabed system and the surface vessel (i.e. the “remote operating station”) is typically via wire and/or cable (eg. electrical or optical fibre telemetry).
Unlike standard CPT test data, SBP test data does not require a correction for pore water pressure. Nevertheless, it would be advantageous for a ball penetrometer to provide a capability to measure this parameter, as this enables dissipation testing to be undertaken at the discretion of the operator, negating the need to complete a second borehole using conventional CPT equipment. Dissipation testing is a standard means of measuring time-dependent soil drainage characteristics. The capability to measure dynamic pore pressure may also be employed to estimate the cyclic performance of soft soils subjected to cyclic loading. Additionally, or alternatively, it would also be advantageous for a ball penetrometer to avoid complications due to relative movement of the soil in contact with the shaft, which normally gives rise to friction forces additional to the bearing forces acting on the ball.
This identifies a need for an improved ball-type penetrometer and method of use thereof which overcomes or at least ameliorates problems inherent in the prior art.
The reference to any prior art in this specification is not, and should not be taken as, an acknowledgment or any form of suggestion that such prior art forms part of the common general knowledge.